Anatase TiO2 Thin Films Research Articles

Optimizing deposition parameters and characterizing TiO2 thin films for future memristor applications

Titanium dioxide (TiO2) thin films were deposited on glass substrates using Spray Pyrolysis technique, with variations in multiple deposition parameters. The molarity of the precursor was altered within a small range from 0.09 M–0.15 M. The deposition temperature was systematically adjusted from 200 °C to 400 °C while the ratio between precursor and chelating agent varied between 1:1,1:2 and 1:3. The thickness of TiO2 films were found to be in the range of 216 nm to 14.9 μm. Structural analysis conducted by XRD confirmed the formation of anatase TiO2 thin films. Optical studies using UV-Visible spectrophotometer determined the absorption and indirect bandgap ranging from 299 nm to 326 nm and 3.08 eV to 3.44 eV respectively. Electrical studies carried out evaluated the leakage currents and DC resistivity for all individual films with the input voltage applied from ± 0.5 V to ± 5V. Impedance studies were conducted by varying input voltages from 0.2V–3V for each film, so as to examine the resultant impedance, dielectric constant, dielectric loss, conductivity, admittance and modulus spectra. The obtained results were analysed to optimise the deposition parameters for designing future memristors with specific individual or combined characteristics, such as high ON/OFF, switching speed, endurance and retention.

Anatase to rutile transition in TiO2 thin films: Role of tantalum and oxygen

The present study discusses the optimal solubility limit of tantalum in anatase TiO2 thin films (dopant solubility limit) and its effect on crystal structure transformation to the rutile phase. The study also focuses on the role of lattice oxygen in this structural phase transition. The detailed chemical composition, constitution, and phase of these sputter-deposited thin films are correlated with their electrical and optical properties to understand the disparity between total impurity concentration in these films and actual impurity substitution into the host lattice sites (dopant content). Reduced oxygen vacancies from the oxygen lattice sites and/or Ta substitution in Ti lattice sites are found to hinder the anatase to rutile phase transformation in TiO2 thin films. Our experimental data indicates the solubility limit of Ta being < 5 at% in TiO2. Similar behavior is seen when Ta is replaced with Nb in TiO2 thin films. Moreover, an increased oxygen and/or impurity concentration is found to lead to impurity-rich-oxides in these films, degrading the electrical conductivity.

Maximizing Quantum Hole Collection on Anatase TiO2 Thin Films with (001) Facets for Photoelectrochemical Oxigen Evolution Reaction Efficiency

Hydrogen production from water, a key element in the renewable energy portfolio, offers a solution to reduce reliance on fossil fuels. Photoelectrochemical (PEC) hydrogen production, using light-absorbing semiconductors and catalysts, offers a clean and cost-effective method for harnessing solar energy. To enhance the efficiency of PEC, this study explores the use of anatase TiO2 thin film photoelectrodes as an alternative to expensive and rare catalysts, aiming to generate hydrogen as a clean fuel with oxygen as a byproduct.Anatase titanium dioxide is a well-established functional material widely applied in the fields of environment and energy. Recognized as one of the premier materials for photocatalysis and a promising photoanode material for water splitting, titanium dioxide (TiO2) finds use in diverse applications such as dye-sensitized solar cells, perovskite-based solar cells, and cleaning devices. Its band gap of 3.2 eV, stability, strong absorption in the UV region, and energetics of the valence and conduction band edges make anatase particularly suitable for PEC water splitting, especially for the oxygen evolution reaction (OER).Our research focuses on the development of anatase thin films with strong [001] texture and (001) facets at the surface of the film. The (001) facets are known to accumulate photogenerated hole states while the [001] orientation imparts a negative flat band potential. We hypothesize that these two properties, (001) facets and [001] texture, are ideal for OER. This study combines these two structural characteristics within the semiconductor TiO2 layer to enhance PEC performance and maximize the efficiency of hole collection during water oxidation.Anatase TiO2 thin films with (001) facets and [001] orientation were hydrothermally synthesized on a fluorine-doped tin oxide (FTO) substrate and annealed at 500°C. The films exhibited [001] orientation and (001) faceting as shown by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Photoelectrochemical measurements via cyclic voltammetry in a 0.1 M KOH and Na2SO3 solution demonstrated the films' hole collection efficiency under a 365 nm UV light source. The photoelectrochemical (PEC) and oxygen evolution reaction (OER) characteristics of anatase thin films were observed both under dark and light conditions. Prior to illumination, only the Ti3+/Ti4+ redox couple was observed with an onset potential of 0.01 V (RHE) and negligible OER at anodic potentials. When UV irradiated, the TiO2 thin films exhibited a photocurrent that increased through the anodic scan. The onset potential of photocurrent, measured in the presence of the hole scavenger, exhibited a negative shift ranging from -100 to -200 mV compared to the KOH onset potential. This shift is attributed to the film surface's limited efficiency in collecting holes for water oxidation at low potentials, as rapid recombination kinetics compete with water oxidation. Interestingly, the photocurrent observed with Na2SO3 electrolyte was 0.15 to 0.3 times more than what we saw with KOH electrolyte. Our preliminary results indicate a current density of ~2.0-2.5 mA cm-2 at 1.8 V and a hole collection efficiency of ~84 % at 1.23 V vs RHE, the thermodynamic water oxidation potential. In summary, we have made significant progress toward enhancing the PEC activity of anatase TiO2 thin films by optimizing the film structure for both reactivity and charge transport.

Crystalline as-deposited TiO 2 anatase thin films grown from TDMAT and water using thermal atomic layer deposition with in situ layer-by-layer air annealing

We report a new thermal atomic layer deposition (thermal-ALD) process including an air exposure as a third precursor to deposit crystalline TiO2 anatase thin films from tetrakis(dimethylamido)titanium(IV) (TDMAT) and water at deposition temperatures as low as 180 °C and film thicknesses as low as 10 nm. This ALD process enables TiO2-antase crystal growth during the deposition at low temperatures (&amp;lt; 220 °C). This additional oxidant pulse is used to fully oxidize the Ti to a 4+ state in the amorphous film, lowering the barrier to crystalline anatase formation. This new approach is informed by preliminary studies of post-deposition annealing (PDA) of thermal ALD films in both nitrogen and air atmospheres, which demonstrate the importance of having an oxidizing atmosphere to achieve the nucleation of the crystalline anatase phase. This oxidizing atmosphere is subsequently introduced into the ALD cycle as a third precursor and is shown to be more effective and efficient in promoting the crystalline transformation than even by post-deposition annealing. The crystalline anatase phase is verified by Raman spectroscopy and grazing incidence X-ray diffraction (GIXRD). The mechanism for crystallization during the TDMAT/H2O/air ALD cycle is probed by chemical state analysis via X-ray photoelectron spectroscopy (XPS). We propose that sub-oxidation in TiO2 thin films deposited by the thermal-ALD process inhibits crystallization during ALD from TDMAT/H2O chemistry. Scanning electron microscopy (SEM) is used to investigate the microstructure of these TiO2 thin films as a function of thickness (5 nm to 50 nm) and deposition temperature (180 °C to 220 °C). The reported layer-by-layer air anneal process is found to crystallize entire films in shorter total process times than thermal-ALD with ex situ post deposition annealing at identical temperatures, presumably due to the improved surface diffusion kinetics accessed during the deposition process.

Transparent Conducting TiO2 Thin Film Induced by Electric‐Field Controlled Hydrogen Ion Intercalation

AbstractRealizing transformation from transparent insulating to transparent conducting, is a pursuing goal in designing and fabricating novel optoelectronic materials and devices. Here, a pronounced insulating to metal transition in anatase TiO2 thin films is achieved through ionic liquid gating, and interestingly the material maintains an invariable high optical transparency. It is revealed that the emergent metallic state can be attributed to the electron doping associated with the hydrogen ion intercalation. Importantly, the hydrogenation leads to the almost rigid shift of the Fermi energy and therefore maintains nicely the transparency at the visible light region. This result is in strong contrast with the case of oxygen vacancy doping, in which the optical bandgap is suppressed due to improved orbitals hybridization and intraband transition. Moreover, through synergistic ion‐electron doping, the selective control of the gating area and pattern is realized in the micrometer or even nanometer scale with extremely distinct physical properties, which can be employed to fabricate novel optical and electronic devices. The result greatly deepens the understanding of the underlying physics and formation mechanism of transparent conducting oxide (TCO) materials. It is envisioned that this work would provide a new pathway to design other potential optoelectronic materials with novel functionalities.

Fabrication of iron doped titanium dioxide photocatalytic nanocomposite supported by water-quenched blast furnace slag and photocatalytic degradation of wastewater

Metallurgical solid waste blast furnace water quenched slag (WBFS) was employed as carrier materials to prepare Fe–TiO2/WBFS composite photocatalyst using a sol-gel method for improving both the photocatalytic activity and the high-value utilization of metallurgical solid waste. The Fe3+ doping technology was adopted to increase the utilization of solar energy. The influencing factors and photocatalytic activity were systematically evaluated by thermogravimetry/differential thermal analyser (TG-DSC), X-Ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) method, scanning electron microscopy-energy spectrometer (SEM-EDS), X-ray photoelectric spectroscopy (XPS), UV–Vis absorption spectrum and photoluminescence spectra technology through the degradation rate of methylene blue (MB) solution. The results showed that the Fe3+ ions were successfully incorporated into the TiO2 lattice, which had no effect on the crystalline phase of TiO2 and expanded the spectral response range of TiO2 from the ultraviolet region to the visible light region. When the calcination temperature was at 450 °C and the number of Fe–TiO2 sol loading cycles was two times, the photocatalytic activity of Fe–TiO2/WBFS presented the strongest, and the degradation ratio of MB reach the maximum value of 98.9 %. A uniform and dense anatase TiO2 thin film was wrapped on the surface of WBFS. The photocatalytic activity first improved and then weakened with an increase of calcination temperature, Fe3+ doping amounts and the loading cycles of sol. The TiO2 film changed from thin to thick and uneven to uniform, until cracking and spalling phenomenon finally occurred with an increase of Fe–TiO2 sol loading cycle from 1 to 3 times. When the Fe–TiO2/WBFS was reused for four times, the degradation rate of MB could still show 67.4 %, being far higher than that of TiO2/WBFS of 25.4 %.

Plasma-Induced 2D Electron Transport at Hetero-Phase Titanium Oxide Interface.

Interfaces of metal oxide heterojunctions display a variety of intriguing physical properties that enable novel applications in spintronics, quantum information, neuromorphic computing, and high-temperature superconductivity. One such LaAlO3 /SrTiO3 (LAO/STO) heterojunction hosts a 2D electron liquid (2DEL) presenting remarkable 2D superconductivity and magnetism. However, these remarkable properties emerge only at very low temperatures, while the heterostructure fabrication is challenging even at the laboratory scale, thus impeding practical applications. Here, a novel plasma-enabled fabrication concept is presented to develop the TiO2 /Ti3 O4 hetero-phase bilayer with a 2DEL that exhibits features of a weakly localized Fermi liquid even at room temperature. The hetero-phase bilayer is fabricated by applying a rapid plasma-induced phase transition that transforms a specific portion of anatase TiO2 thin film into vacancy-prone Ti3 O4 in seconds. The underlying mechanism relies on the screening effect of the achieved high-density electron liquid that suppresses the electron-phonon interactions. The achieved "adiabatic" electron transport in the hetero-phase bilayer offers strong potential for low-loss electric or plasmonic circuits and hot electron harvesting and utilization. These findings open new horizons for fabricating diverse multifunctional metal oxide heterostructures as an innovative platform for emerging clean energy, integrated photonics, spintronics, and quantum information technologies.

Swift heavy ion irradiation induced modifications on the structure, optical and electrical properties of pulsed laser deposited anatase TiO2 thin films

The swift heavy ions of Ag7+ (100 MeV) were used to irradiate the anatase TiO2 thin films prepared by pulsed laser deposition (PLD) technique on quartz substrates. The effect of ion fluences (1×1012, 1×1013 and 1×1014 ions/cm2) on the structure, surface morphology, optical and electrical properties are investigated through several analytical techniques such as XRD, Micro-Raman, FTIR, XPS, AFM, FESEM, UV–visible and Hall measurements. The analysis of XRD data shows a gradual shift in the (101) plane toward higher diffraction angle on increasing the ion fluence which increases the lattice strain in the films. The EDS spectra reveal a modest Ti-rich composition with a significant shift following irradiation for both as-grown and irradiated films. The development of hillocks at increased ion fluence and considerable grain splitting following irradiation are observed in SEM micrographs. AFM images exhibit increase in surface roughness with the increase in ion fluence. Moreover, there is a significant drop in the band gap of the irradiated thin films from 3.92 to 3.44 eV with the increase in ion fluence. The various electrical parameters such as carrier concentrations (n), Hall mobility (μ), electrical resistivity (ρ), Hall coefficient (RH), etc., are determined by the Hall measurements at room temperature. This study indicates that the SHI irradiation plays a significant role in tunning the physical properties of anatase TiO2 thin films.

Tailoring Black TiO2 Thin Films: Insights from Hollow Cathode Hydrogen Plasma Treatment Duration

In this study, we report the use of a radiofrequency plasma-assisted chemical vapor deposition (RF-CVD) system with a hollow cathode geometry to hydrogenate anatase TiO2 thin films. The goal was to create black TiO2 films with improved light absorption capabilities. The initial TiO2 was developed through magnetron sputtering, and this study specifically investigated the impact of hollow cathode hydrogen plasma (HCHP) treatment duration on the crucial characteristics of the resulting black TiO2 films. The HCHP treatment effectively created in-bandgap states in the TiO2 structure, leading to enhanced light absorption and improved conductivity. Morphological analysis showed a 24% surface area increase after 15 min of treatment. Wettability and surface energy results displayed nonlinear behavior, highlighting the influence of morphology on hydrophilicity improvement. The anatase TiO2 phase remained consistent, as confirmed by diffractograms. Raman analysis revealed structural alterations and induced lattice defects. Treated samples exhibited outstanding photodegradation performance, removing over 45% of methylene blue dye compared to ~25% by the pristine TiO2 film. The study emphasized the significant impact of 15-min hydrogenation on the HCHP treatment. The research provided valuable insights into the role of hydrogenation time using the HCHP treatment route on anatase TiO2 thin films and demonstrated the potential of the produced black TiO2 thin films for photocatalytic applications.

Electrochemical Lithium Insertion into TiO2 Anatase ALD Thin Films for Li‐Ion Microbatteries: An Atomic‐Scale Picture Provided by Raman Spectroscopy

AbstractElectrochemical lithium (de)intercalation in an atomic layer deposited (ALD) TiO2 anatase thin film deposited on a planar Si /Al2O3/Pt substrate is investigated by Raman spectroscopy. An initial discharge capacity of 63 µAh cm−2 µm−1 (0.5 Li+ mole−1) is reached at C/10 rate, which increases up to 77 µAh cm−2 µm−1 upon further cycles. An excellent capacity retention is achieved over at least 100 cycles, showing the good adherence of the ALD thin film. Raman spectra of LixTiO2 (0 ≤ x ≤ 0.5) thin film electrodes point to the nucleation of the orthorhombic lithiated titanate (LT) Li0.5TiO2 phase from x = 0.1. This LT phase coexists with tetragonal TiO2 in the 0.1 ≤ x ≤ 0.4 composition domain to be pure for x = 0.5. A fully reversible transformation from orthorhombic LT to tetragonal TiO2 is observed upon the charge. The high quality of the Raman spectra allows identifying for the first time 12 modes in the 100–800 cm−1 region for the electrochemically formed LT phase. Furthermore, an appropriate Raman spectra analysis allows a reliable and quantitative determination of the thin film composition during discharge and charge. These results illustrate Raman spectroscopy is a powerful probe to scrutinize the Li insertion/extraction mechanism in TiO2 thin films.

Effect of TiCl4 treatment time on the properties of anatase TiO2 thin films synthesized by spray pyrolysis technique

Effect of TiCl4 treatment time on the properties of anatase TiO2 thin films synthesized by spray pyrolysis technique

Single Atoms in Photocatalysis: Low Loading Is Good Enough!

We disperse Pt single atoms (SAs) with different loading densities on anatase TiO2 thin films and evaluate the photocatalytic H2 generation as a function of the incident light intensity. We show that under common illumination intensities (such as terrestrial solar illumination), a minuscule Pt SA loading of ∼105 atoms μm–2 (surface Pt content ∼0.1 at.%) is sufficient to achieve a maximized H2 production rate. This results in a maximum turnover frequency at a single Pt atom site of ∼300 H2 molecules s–1. For a vast majority of illumination conditions and a suitable surface configuration, it is not the density of co-catalytic sites that is rate-determining (a high loading is not needed!), but the charge carrier generation (and flux of photoelectrons to the co-catalytic centers) determines the overall reaction rate. This is in stark contrast to SA catalysis of classic chemical reactions where generally a maximum loading of reactive SA delivers a maximum activity.

Photovoltaic performances of TiO2/Se heterojunction devices with different crystallographic structures of sputter-deposited TiO2 thin films

This study investigates the impact of sputter-deposited TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices, one of the most promising indoor photovoltaic energy harvesters and wide-gap top cells in tandem solar cells. Since incident light illuminates from TiO2 n-type window layer of TiO2/Se photovoltaic devices, most of photons carriers is absorbed in the Se layer. It is essential for highly efficient photovoltaic conversion of TiO2/Se devices to improve carrier collection by adequate formation of built-in potential in the Se layer and to reduce recombination by optimizing energy band alignment at TiO2/Se interface. In this study, TiO2 window layers with different contents of anatase and rutile crystallographic structures are applied to TiO2/Se heterojunction devices. Impacts of TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices are investigated in conjunction with different photoconductivity and energy band structures between anatase and rutile TiO2 thin films. In TiO2/Se photovoltaic devices with anatase/rutile mixture window layer, the power conversion efficiency was increased from 3.04% to 4.48% by combining the high open circuit voltage and high short circuit current, owing to the smaller cliff-type conduction band offset at −0.63 eV and smaller series resistance resulting from fitting the current density–voltage curves.

Porosity-dependent photoelectrochemical activity of double-layered TiO2 thin films deposited by spin-coating method.

Photoelectrochemical (PEC) cells made of low-cost, chemically stable, and abundant materials are crucial for green hydrogen production. In this regard, the fabrication of porous films with high light trapping ability and a large contact area is crucial for the production of efficient PEC cells. In this report, anatase TiO2 thin films with a porous double-layered structure were successfully prepared using a conventional spin-coating deposition method. Various amounts of polystyrene spheres were used as a pore-templating agent to control the porosity of the films. A range of characterization techniques, such as scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and photoluminescence were employed to assess the morphology, structural and optical properties of prepared TiO2 films. PEC measurements revealed that prepared double-layered TiO2 thin films exhibit porosity-dependent photocatalytic activity. For example, TiO2 films with an optimized porous structure demonstrated an increase in photocurrent density by a factor of ∼2.23 (to 141.7 μA cm-2) and photoconversion efficiency improvement by a factor of ∼2.14 as compared to non-porous double-layered TiO2 reference films. Absorbance and photoluminescence analysis confirmed that improved PEC activity can be attributed to increased light absorption by the porous structure and reduced charge carrier recombination.

A New Study on the Effect of Pure Anatase TiO2 Film Thickness on Gentian Violet Photodegradation Under Sunlight: Considering the Effect of Hole Scavengers

In this paper, pure anatase TiO2 thin films were grown on glass substrates by using sol-gel dip-coating method. To study the physical, chemical and optical properties of the deposits, TiO2 thin films were synthesized with thickness (241.5, 258.19, 230.33 and 302.35 nm). XRD patterns show that TiO2 films were grown with unique anatase phase (101). The photocatalytic test shows that film (~ 302 nm) gives high photodegradation of gentian violet (GV) under sunlight irradiation due to the low number of defect sites. The increase in film thickness and the decrease of surface roughness and crystal size decrease the number of defect sites in the material which lead to low recombination rate of charge carriers. The use of Ag+ as hole scavenger increases the photocatalytic activity 4.75 times within the first hour. The photocatalysis process showed that Ag+ can be reduced on the surface of photoactivated TiO2 thin films to obtain sun-photodeposited/dip-coated AgO/TiO2 (p-n) heterojunction. HIGHLIGHTS Pure anatase TiO2 thin films were successfullysynthesized via sol-gel dip-coating method High thickness (302 nm) of TiO2 thin film exhibits high photocatalytic activity for photodegradation of GV under sunlight irradiation The presence of Ag+ in wastewater shows high photocatalytic activity of TiO2 films with 4.75 times within the first hour The possibility of sun-photodeposited/dip-coated of (p-n) AgO/TiO2 heterojunction is discussed GRAPHICAL ABSTRACT

Effect of annealing temperature on the microstructure and optoelectronic properties of niobium-doped anatase TiO2 thin films grown on soda-lime glass substrate

Effect of annealing temperature on the microstructure and optoelectronic properties of niobium-doped anatase TiO2 thin films grown on soda-lime glass substrate

Structural and photoelectrochemical dynamics of in-situ hydrogenated anatase TiO2 thin films grown by DC reactive magnetron sputtering

Hydrogenation has become one of the most used strategies to improve the photoactive properties of titanium dioxide nanomaterials, n-TiO2. To obtain more information about the hydrogenated process on anatase TiO2 thin films and the improvement of the photoactivity of this material, we report a study on the structural changes of hydrogenated anatase thin films produced by direct-current reactive magnetron sputtering. In the first stage of the hydrogenation process, the obtained polycrystalline anatase films present an increment of the {001} facet according to the amount of hydrogen used in the plasma reaction. At higher hydrogen concentrations, amorphous and rutile phases start to appear. The photoactivity of the hydrogenated anatase samples, H:TiO2, presents a redshift of the photoelectrochemical onset and an increase of the reactivity in the UV region. Both results were experimentally and theoretically related to the formation of the {001} facet and the TiH/TiOH bonds at the surface of the hydrogenated thin films.

Synthesis Control of Charge Separation at Anatase TiO2 Thin Films Studied by Transient Surface Photovoltage Spectroscopy.

For the efficient photocatalytic oxidation of organic pollutants at surfaces of semiconductors, photogenerated holes shall be separated toward the surface and transferred to reactive surface sites, whereas the transfer of photogenerated electrons toward the surface shall be minimized. In this Research Article, the identification of suitable synthesis control of charge separation combined with an in-depth understanding of charge kinetics and trapping passivation mechanisms at the related surfaces can provide tremendous opportunities for boosting the photocatalytic performance. In this work, a comprehensive transient surface photovoltage spectroscopy study of charge separation at anatase TiO2 thin films, synthesized by ultrasonic spray pyrolysis from titanium(IV) isopropoxide (TTIP)–acetylacetone (AcacH) based precursor is reported. By varying the amount of AcacH in the precursor solution, an experimental approach of synthesis control of the charge transfer toward TiO2 surface is provided for the first time. An increased amount of AcacH in the precursor promotes transition from preferential fast electron to preferential fast hole transfer toward anatase surface, correlating with a strong increase of the photocatalytic decomposition rate of organic pollutants. Suitable mechanisms of AcacH-induced passivation of electron traps at TiO2 surfaces are analyzed, providing a new degree of freedom for tailoring the properties of photocatalytic systems.

Correlation between optical and structural properties of nitrogen doped anatase TiO2 thin films

Raman spectroscopy, grazing incidence X-ray diffractometer (GIXRD), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and photoluminescence spectroscopy (PL) were performed on TiO2 thin films exposed to a nitrogen N2+ ion beam at different temperatures from 45 °C to 585 °C. The crystalline structure, the thickness, the dielectric functions, the band gap and the Urbach energies were studied. The obtained results show a strong change of physical properties of TiO2 after exposing to N2+ ion beam. Raman modes and diffracted peaks show the anatase structure for all the deposited N-doped films. The presence of nitrogen doping into the TiO2 layer is confirmed by XPS measurements. The substitutional nitrogen defects promote a decrease of the band gap for N-doped TiO2 by 150 meV at the doping temperature 585 °C. This reduction was explained by the increase in valence band extremity caused by the hybridization of the N 2p states and the O 2p states of the valence band of TiO2. The Raman, GIXRD, XPS, SE, AFM, and PL results are consistent and provide an invaluable complementary information about the nitrogen diffusion within the TiO2 lattice at different temperatures.

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2 Thin Films Grown by Atomic Layer Deposition

The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600–800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile–brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250–500 °C that allows utilization in broad range of TiO2 thin film applications.